Add traits for sign of units and magnitudes

[chiphogg]

The "sign" of a unit or magnitude is always mag<1>() (i.e.,
Magnitude<>{}) if positive, or -mag<1>() (i.e.,
Magnitude<Negative>{}) if negative.

We also update the docs, including adding docs for a few new traits that
we had missed, and updating the headings for the magnitude traits to
phrase the boolean ones in the form of a question (in case we ever get
on Jeopardy).

Helps #428.